Various techniques for forming fine photoresist patterns have been proposed for the purpose of realizing highly integrated semiconductor devices in the field of photolithography. The techniques employ the method of projecting light for exposure. The phase shifting exposure method is one attractive fine pattern forming method.
The phase shifting method is, for example, disclosed in Japanese Patent Publication No. 62-59296.
A conventional phase shifting mask will be described hereinafter with reference to FIG. 1. In FIG. 1, an array of chromium patterns 22 is formed on a mask substrate 21. The chromium patterns 22 are stripe-shaped and arranged respectively at a given pitch. The areas on which the chromium patterns 22 are formed are opaque, and others are light-transmissive areas.
Thin films (phase shifters) 23 for shifting the phase of light are formed on every other light-transmissive area and between the chromium patterns 22. The widths of the chromium pattern L.sub.1 are equal to those of the light-transmissive spaces S.sub.1 between the chromium patterns 22. The phase shifter improves the contrast of light and the resolution of the pattern image formed on a wafer.
However, when the photomask is employed to form a positive type resist pattern, unnecessary patterns ,may not be completely removed due to the transfer of the edges 24 of the phase shifters 23 on light-transmissive areas. This problem is disclosed in PROCEEDINGS OF THE 37TH SYMPOSIUM ON SEMICONDUCTORS AND INTEGRATED CIRCUITS TECHNOLOGY, 1989, pp 13 to 16.
On the other hand, a measure for the transfer prevention is disclosed in EXTENDED ABSTRACTS OF THE 51st AUTUMN MEETING, 1990, THE JAPAN SOCIETY OF APPLIED PHYSICS, Lecture No. 27p-ZG-6, p. 492.
In the conventional phase shifting mask, if the edge of the phase shifter exists in a light-transmissive area, the intensity of light at the position corresponding to the edge on the wafer is reduced to zero. According to the proposal of the above literature, a film with a thickness corresponding to the phase difference of 90.degree. (sub-shifter) is formed at the edge of the phase shifter and thereby the decrease in intensity of light is moderated so as to prevent the transfer of the edge of the phase shifter.
This method, however, needs at least two kinds of shifters having different thickness thereby, complicating the manufacturing process and increasing the manufacturing cost.